D-tubocurarine blocks the neuromuscular transmission by
**Core Concept**
D-tubocurarine is a non-depolarizing neuromuscular blocking agent that acts by competitively blocking the nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction. This leads to a failure of acetylcholine to produce muscle contraction, resulting in muscle paralysis.
**Why the Correct Answer is Right**
D-tubocurarine binds to the nicotinic acetylcholine receptors on the muscle endplate, competing with acetylcholine for receptor binding sites. This competitive inhibition prevents acetylcholine from activating the muscle fibers, leading to muscle weakness and paralysis. The receptor blockade is reversible, but the duration of action depends on the dose and concentration of the drug.
**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect, as this does not accurately describe the mechanism of action of d-tubocurarine.
* **Option B:** Incorrect, as this is the mechanism of action of depolarizing neuromuscular blocking agents like succinylcholine, not non-depolarizing agents like d-tubocurarine.
* **Option C:** Incorrect, as this is the mechanism of action of competitive inhibitors of acetylcholinesterase, like neostigmine, which increase the concentration of acetylcholine at the neuromuscular junction.
**Clinical Pearl / High-Yield Fact**
Non-depolarizing neuromuscular blocking agents like d-tubocurarine have a longer duration of action and a slower onset compared to depolarizing agents like succinylcholine, making them useful for procedures requiring prolonged muscle relaxation.
**Correct Answer: B. Blocking of nicotinic acetylcholine receptors at the neuromuscular junction.**